1. Field of the Invention
This invention relates to a video camera equipped with an anti-shake system, that is, a system for correcting a camera-body shake usually caused by user's hand. In addition, this invention relates to a method of processing an information signal such as a shake detection signal.
2. Description of the Related Art
A first prior-art anti-shake system for a video camera includes a shake sensor and a controller. The shake sensor detects a shake of the body of the camera which is caused by user's hand. The shake sensor outputs a signal representative of the detected camera shake. An electric circuit in the camera generates a video signal from an electric signal outputted by an image sensor. The controller processes the video signal in response to the output signal of the shake sensor to compensate for the camera shake.
A second prior-art anti-shake system for a video camera includes an active prism, angular velocity sensors, and a controller. The active prism is provided in an optical system of the camera. The active prism functions to move the optical axis of the optical system relative to a CCD image sensor of the camera. The angular velocity sensors detect the angular velocities of the body of the camera in the vertical direction and the horizontal direction, respectively. The angular velocity sensors output signals representative of the detected angular velocities to the controller. The controller drives the active prism in response to the output signals of the angular velocity sensors, thereby compensating for a shake of the camera body which is caused by user's hand. As a result of the compensation, the position of the image of a subject on the CCD image sensor remains substantially fixed independent of the camera-body shake.
Japanese patent application publication number 7-288734/1995 discloses a camera apparatus equipped with an anti-shake system. The camera apparatus in Japanese application 7-288734/1995 includes angular velocity sensors which detect the angular velocities of the body of the camera apparatus in the pitching direction and the yawing direction respectively. The angular velocity sensors generate sensor signals representing the detected angular velocities. The sensor signals are fed to an A/D converter via filters and amplifiers. The A/D converter changes the sensor signals into first digital data and second digital data representing the detected angular velocities. Low pass filters extract drift components from the first and second digital data. The extracted drift components are fed to a pan tilt discrimination circuit as signals indicative of the center values (the 0 levels) related to the first and second digital data. The pan tilt discrimination circuit sets threshold values for the first and second digital data while using the 0 levels as references. The pan tilt discrimination circuit compares the levels represented by the first and second digital data with the threshold values. The pan tilt discrimination circuit decides whether panning the body of the camera apparatus or a shake thereof caused by user's hand is occurring, and whether tilting the body of the camera apparatus or a shake thereof caused by user's hand is occurring on the basis of the results of the comparison. High pass filters remove direct-current components from the first and second digital data, respectively. The high pass filters have cut-off frequencies. The pan tilt discrimination circuit varies the cut-off frequencies of the high pass filters in response to the result of the decision about whether panning the body of the camera apparatus or a shake thereof caused by user's hand is occurring, and in response to the result of the decision about whether tilting the body of the camera apparatus or a shake thereof caused by user's hand is occurring. The output signals from the high pass filters are integrated by integration circuits. A comparator circuit receives the integration-resultant signals as indications of the detected angular velocities in the pitching and yawing directions.
The camera apparatus in Japanese application 7-288734/1995 further includes an active prism. Pitching-direction motion of the active prism is detected by a first braking coil and a first photosensor. Signals representative of the detected pitching-direction motion are fed to the comparator circuit. Yawing-direction motion of the active prism is detected by a second braking coil and a second photosensor. Signals representative of the detected yawing-direction motion are fed to the comparator circuit. The comparator circuit compares the integration-resultant signals, the signals representative of the detected pitching-direction motion, and the signals representative of the detected yawing-direction motion. The comparator circuit controls a pitching-direction drive system and a yawing-direction drive system for the active prism in response to the results of the comparison. Thereby, it is possible to accurately compensate for a hand-caused shake of the body of the camera apparatus. In addition, it is possible to discriminate panning the body of the camera apparatus, tilting the camera-apparatus body, and a hand-caused shake of the camera-apparatus body from each other.
In the case of an anti-shake system including angular velocity sensors and an A/D converter for changing the output signals of the angular velocity sensors into corresponding digital signals, the dynamic range and accuracy of the system are basically decided by the A/D converter for the reasons as follows. The A/D converter periodically samples the sensor signals to generate analog signal samples. The A/D converter quantizes the analog signal samples in response to a quantization step size to generate digital signal samples each having a predetermined number of bits. The inevitable quantization error decreases and hence the accuracy of the digital signal samples rises as the quantization step size decreases. The dynamic range of the A/D converter widens as the bit number per digital signal sample increases. A decrease in the quantization step size and an increase in the bit number per digital signal sample cause an increase in the circuit scale of the A/D converter. In addition, an increase in the bit number causes a reduction in the rate of the processing of the digital signal samples.